1. Field of the Invention
This invention relates to attenuators for use with optical fibers.
2. Discussion of the Known Art
Signals carried by optical fibers require attenuation for various purposes, including but not limited to optical signal equalization of separate voice or data channels that are coupled to an optical multiplexing or de-multiplexing device. Attenuation is also necessary to reduce the level of optical signals at a receiving device, e.g., to keep the signals within a linear operating range of the device. Attenuators can also be used in optical sensing elements and optical actuators. All of the foregoing instances requiring attenuation of optical signals are best implemented with a controllable, variable optical attenuator device.
It is also desirable to incorporate a variable optical attenuator with so-called xe2x80x9cjumpersxe2x80x9d that are found typically near optical transmitter and receiver devices such as, e.g., cable TV laser transmitters at a network operations center, a SONET multiplexer at a central office or a remote cabinet, or a data network switch or hub in an equipment room for a local area network (LAN). Jumpers are also used to connect a desk top computer or server to a wall outlet in a LAN, and for inter-connecting and cross-connecting indoor building equipment with outside plant cables. While optical fiber jumpers are typically of short length (about 2 to 10 meters), some may be as long as 300 meters. Jumpers take the form of fiber optic cables with optical connectors such as types ST, SC or LC, on each end of the cable. A jumper cable typically has a central glass optical fiber, an ultra-violet (UV) cured coating, a surrounding layer of buffer material, aramid yarn around the buffer layer, and a protective outer cable jacket.
Basically, there are four techniques for attenuating a light signal propagating within an optical fiber with some degree of control; namely, absorption, scattering, macrobending, and microbending.
Absorption and scattering of light are used typically in fixed optical attenuators, but it is difficult to manufacture such attenuators to obtain small attenuation values. Also, return losses greater than xe2x88x9260 dB are common.
Macrobending of optical fibers in loops may also produce attenuation, but requires very small bend radii to achieve large levels of attenuation. And, the presence of small bend radii leads to excessive stress on the fibers, causing mechanical damage when held in the fibers for long periods of time.
Microbending of optical fibers by clamping mechanical gratings having teeth or other periodic undulations about the fiber, has also been used for signal attenuation as discussed below. With such gratings, optical fibers can be forced to attenuate light guided within them when the mechanical grating period corresponds to a resonant coupling period of guided to non-guided light propagation modes in the fiber. See D. Marcuse, Theory of Dielectric Optical Waveguides, Academic Press (2d ed. 1974) at pages 83-84, 134-39, and 157-58, all of which is incorporated by reference. For an optical fiber of 250 microns diameter (including UV coating but without buffer material), the fundamental mechanical grating periods are typically about 350 microns for a single-mode fiber, and about 1,000 microns for a graded-index multi-mode fiber. For fibers that are coated with a thick buffer layer so that the overall diameter of the composite fiber is about 900 microns, mechanical gratings having such periodicity will require a relatively large clamping force to induce attenuation in the fiber, because of the thick buffer layer surrounding the fiber, however.
Variable optical attenuators for single or multi-mode optical fibers are known in the form of devices arranged to be connected in-line with an existing fiber optic cable, through cable connectors mounted on the device, For example, devices are offered by Net Optics which operate by adjustably mis-aligning optical fibers within the device, or by moving a threaded screw to block a collimated beam between two lenses in the device. Variable optical attenuators offered by Siecor as complete cable assemblies with standard connectors at each cable end, have an attenuator device integrated along the length of the cable. Rotation of a screw on the device operates to vary the position of a gradient filter within a light beam in the device.
As mentioned above, it is known that light signals passing through an optical fiber may be attenuated by application of an external clamping mechanism to induce periodic microbends in the fiber, thus allowing some of the light signal energy carried by the fiber to be lost, e.g. by radiation peripherally out of the fiber. See, e.g., L. T. Wood, et al, Optical Attenuation by Periodic Microdistortions of a Sensor Fiber, Optics Letters (December 1985), at 632-34; and M. B. J. Diemeer, et al, Fiber-Optic MicrobendSensors: Sensitivity as a Function of Distortion Wavelength, Optics Letters (June 1984), at 260-62. See also U.S. Pat. No. 4,749,248 (Jun. 7, 1988) and U.S. Pat. No. 3,931,518 (Jan. 6, 1976). Both of these patents disclose clamp-on devices in the form of opposed, corrugated plates that are clamped about a fiber to achieve a periodic axial distortion of the fiber for purposes of mode coupling.
The known clamping techniques are directed to non-buffered optical fibers, however. As mentioned, buffered optical fibers of the kind used in jumper cables typically comprise an optical fiber (core, cladding and UV coating) with a diameter of about 250 microns, and a surrounding protective layer of buffer material (e.g., nylon, PVC or PE) having an overall diameter typically of about 900 microns. Because of the relatively large clamping force needed to induce a certain degree of microbending in a buffered optical fiber, with respect to the force needed if the same grating was used to induce the same amount of microbending in an unbuffered fiber, it was not believed practical to apply a grating on jumper cables with buffered optical fibers for the purpose of obtaining a controlled amount of light signal attenuation.
According to the invention, a method of attenuating signals propagating in an optical fiber of a first diameter and having a buffer material disposed about the fiber to form a buffered optical fiber having a second diameter greater than said first diameter, includes determining a fundamental grating period that would induce attenuation of signals propagating in the optical fiber in the absence of the buffer material, and clamping a grating along a certain axial length of the buffer material on the fiber. The grating is formed with a period that is N times the fundamental grating period for the optical fiber, wherein N is a positive integer.